1. Field of the Invention
The present invention relates generally to a parallel operation system of transmission amplifiers, and more particularly to transmission amplifier parallel operation system adapted for a mobile radio base station.
2. Description of the Related Art
With mobile radio communication terminals prevailing in recent years and furthermore demands for high-speed data services rising, increasing the transmission output of base stations has become a greater problem. To increase the transmission output of base stations, high-powered transmission amplifiers are needed and to increase the electric power of transmission amplifiers it is generally necessary to enlarge heat dissipation fins. Therefore it is unavoidable that the configuration of transmission amplifiers becomes big.
On the other hand, when transmission amplifiers are made to perform amplification functions in the nonlinear range of amplification characteristics to obtain high-efficient output, nonlinear distortions arise. To compensate it, feed forward system is generally adopted.
As disclosed in Japan Patent Pub. No. Hei7-77330, for example, the transmission amplifiers adopting the feed forward system are configured such that distortion components are found from the difference between main amplifiers' inputs and outputs, the found distortion components are amplified to the level equivalent to the outputs of the main amplifiers and then added to the outputs of the above main amplifiers, and the found distortion components are canceled. Amplifier outputs with compensated distortions can thus be obtained.
However, the configuration of the transmission amplifiers adopting the feed forward system needs amplifiers for distortion components that amplify the distortion components to the level equivalent to the outputs of the main amplifiers. A delay circuit is also needed to adjust the timing to add the outputs of the amplifiers for distortion components and those of the main amplifiers.
Furthermore, there is a digital pre-distorter system as a method to compensate the nonlinear distortions of high-powered transmission amplifiers. The principle of the digital pre-distorter system is that characteristic components opposite to the distortion characteristics of the amplifiers are prepared on the input sides of the amplifiers through digital processing and these are added beforehand on the input sides of the amplifiers through digital processing. As a result, the distortion components created by the amplifiers themselves are canceled and the amplified outputs free from any distortions can be obtained from the amplifiers.
A configuration example of amplifiers using the digital pre-distorter system has been proposed before by the present applicant (see Japan Patent Laid-open Pub. No. Hei9-69733).
The amplifiers using the digital pre-distorter system don't need the amplifiers for distortion components, compared with the configuration that adopts the feed forward system. Therefore the amplifiers can be constituted easily because there is no need of adjustment to time the outputs of the amplifiers for distortion components and those of the main amplifiers.
On the other hand, mobile radio base stations adopt a redundancy configuration of transmission amplifiers to secure the reliability of devices. By way of example, FIG. 1 shows a redundancy configuration of the transmission amplifiers that adopt the feed forward system.
In FIG. 1, two amplifiers 1 and 2, each of which has distortion compensating function, adopt the above feed forward system and are parallel connected for the redundancy configuration. In normal times, transmission signals are modulated in a modulation unit 3 and its output is two branched in a branching unit 4.
The two branched outputs from the modulation unit 3 are inputted into the transmission amplifiers 1 and 2 respectively and amplified to the respective predetermined levels. The outputs of the transmission amplifiers 1 and 2 are again connected and outputted by a coupling unit 5. Through this in normal times, it is possible to obtain the output power for which the output powers of the transmission amplifiers 1 and 2 are combineed in the coupling unit 5.
Also in the configuration shown in FIG. 1, when either of the systems of the transmission amplifiers 1 and 2 is in trouble, the output power is halved but it is possible to continuously keep the output.
Furthermore, in the configuration shown in FIG. 1, when the output is two branched by the branching unit 4 in normal times, the level is halved (−3 dB). But the output of the modulation unit 3 is distributed in common phase in the branching unit 4 and the respective outputs of the transmission amplifiers 1 and 2 are connected in common phase in the coupling unit 5. Through this in the coupling unit 5, the loss in the branching unit 4 at the time of branching can be canceled. Therefore it is possible to use the outputs of the transmission amplifiers 1 and 2 efficiently also in the parallel configuration.
In FIG. 1, one modulation unit 3 is shown. But for the parallel configuration, two modulation units are prepared and either of the outputs is inputted into the branching unit 4 for redundancy.
As explained earlier, compared with the configuration of the distortion compensation amplifiers of the feed forward system, it seems advantageous to employ the transmission amplifiers using the digital pre-distorter system also in parallel running for the redundancy configuration because the configuration is easy in that neither the amplifiers for the distortion components nor the delay circuit and the like are necessary.
On the basis of such an idea, FIG. 2 shows an assumed configuration example when the transmission amplifiers using the digital pre-distorter system are parallel run. In the case of the transmission amplifiers using the digital pre-distorter system, because modulation units 3-1 and 3-2 where transmission signals are branched and inputted are arranged on preceding stages of the main amplifiers 10-1 and 10-2, the transmission amplifiers 1 and 2 that are parallel run and distortion compensated have the following problems.
That is, the modulation units 3-1 and 3-2 are respectively constituted, including digital pre-distorters (DPDs) 30-1 and 30-2, quadrature modulators (QMODs) 31-1 and 31-2, up-converters (UCONVs) 32-1 and 32-2 and down-converters (DCONVs) 33-1 and 33-2.
In FIG. 2, the up-converters (UCONVs) 32-1 and 32-2 have the function to change frequencies by multiplying carrier frequencies from local oscillators 320-1 and 320-2 in multipliers 321-1 and 321-2. Filters 322-1 and 322-2 are roll off filters to limit bands.
As described above, the two distortion compensation amplifiers 1 and 2 that are parallel run retain local oscillators 320-1 and 320-2 individually to change the frequencies in the respective up-converters (UCONVs) 32-1 and 32-2.
However, the phase difference of the output carriers is not compensated concerning the two local oscillators 320-1 and 320-2. Therefore it is difficult to connect in common phase the outputs of the two transmission amplifiers 1 and 2 in the coupling unit 5. So it is not ensured to compensate in the coupling unit 5 the loss when transmission signals are branch inputted into the transmission amplifiers 1 and 2.
When the transmission amplifiers are redundancy configured to secure the reliability of the device and the digital pre-distorter system is adopted for the transmission amplifiers due to the above reasons, the direct application corresponding to the feed forward system in FIG. 1 was difficult.